Linear motors having a flat secondary and a flat, mobile primary(s) may be employed as drive means for elevators. In one embodiment, a rail fixedly mounted in the hoistway acts as the secondary of the linear motor. The primary(s) attaches to and drives either the elevator car or the counterweight, using the rail as both secondary and guide. This elevator drive arrangement advantageously fits within the hoistway, thereby eliminating the need for a separate machine room.
Linear motor drives are not without their problems, however. To begin, standard guide rails typically do not provide enough cross-sectional area to accommodate the magnitude of magnetic flux generated by the motor primary. As a result, the density of the flux in the rail generally exceeds acceptable limits, thereby negatively effecting motor performance.
The problem is exacerbated in multiple motor configurations. For example, Japanese Patent Publication No. 63-117884 by Mitsui teaches a four-motor configuration having two pairs of motors symmetrically opposed to one another. Symmetrically opposed motors offer the advantage of each motor's attractive force balancing the attractive force of the other motor. The disadvantage of symmetrically aligned motors is that the cross-section of the guide rail (or secondary) must be wide enough to accommodate the magnetic flux generated from primaries on both sides of the rail. Wider secondaries are more expensive to both fabricate and to install. In some applications heavier rails may even require that the building be reinforced to accommodate the increased load.
In addition, four-motor configurations as taught by Mitsui are inherently inefficient. All flat linear motors have motor windings which include numerous coil ends extending out from the metallic body of the primary. The shape of each coil end is determined by the smallest geometry possible which still permits the motor windings to be routed through the metallic body of the primary. Since the coil ends extend a length outside the body of the primary, they do not participate in the motor thrust and consequently do not increase the efficiency of the motor. In fact, they create joule losses thereby decreasing the efficiency of the motor.
Mitsui's multi-motor arrangement, with four similar primaries, has the same ratio of primary body width to coil end length as a linear motor with a single primary of similar primary body width. In the four primary arrangement, the amount of body width and coil end length is just a multiple of that in the single primary linear motor. Therefore, whatever inefficiencies are associated with the ratio of primary body width to coil end length in the single primary are present in the four primary arrangement of Mitsui, assuming that all other variables such as the numbers of windings remain constant.
In sum, what is needed is a new multiple linear motor arrangement.